How We Build Calculators
Every thy.finance calculator goes through a rigorous development and testing process to ensure accuracy.
Our Process
1. Research & Planning
We start by researching the financial domain thoroughly:
- Review official formulas from IRS, SSA, and financial institutions
- Study industry standards and best practices
- Identify edge cases and common user scenarios
- Document all assumptions and calculation methods
2. Implementation
Our calculators are built with accuracy as the top priority:
- Proper financial formulas - We use correct time-value-of-money calculations, compound interest formulas, and tax calculations
- Precision handling - All monetary values use proper rounding (to nearest cent) at display time only
- Clear defaults - Sensible default values based on current market conditions
- Client-side processing - Your data never leaves your browser
3. Validation Testing
This is where thy.finance differs from most calculator sites. We run comprehensive automated tests:
580 Validation Tests
Every calculator has 20 validation tests covering real scenarios, edge cases, and boundary conditions. These tests compare our results against Python reference implementations.
424 Unit Tests
Function-level tests ensure each calculation component works correctly in isolation. These catch rounding errors, formula bugs, and logic issues.
Black Box Testing
End-to-end tests simulate real user interactions to verify the complete calculation pipeline works as expected.
Performance Testing
Tests ensure calculators respond quickly, even with complex calculations like Monte Carlo simulations (50,000+ scenarios).
4. Independent Verification
We verify our JavaScript calculators against independent Python implementations:
- Python scripts use the same formulas as our calculators
- Test cases run through both JavaScript and Python
- Results must match within $0.10 (typically within $0.01)
- Any discrepancies trigger investigation and fixes
Testing Standards
Test Coverage by Calculator Type
| Category | Calculators | Validation Tests | Pass Rate |
|---|---|---|---|
| Investment | 5 | 100 | 100% |
| Retirement | 4 | 80 | 100% |
| Savings | 3 | 60 | 100% |
| Housing | 4 | 80 | 100% |
| Loans | 3 | 60 | 100% |
| Business | 4 | 80 | 100% |
| Tools | 5 | 100 | 100% |
| Total | 28 | 580 | 100% |
Financial Accuracy
Formulas We Use
- Time Value of Money - Standard PV, FV, PMT calculations using proper compounding
- Amortization - Correct loan payment schedules with principal/interest breakdown
- Tax Calculations - IRS-standard tax bracket calculations for 401(k), IRA, and Roth IRA
- Investment Returns - Compound annual growth rate (CAGR) and total return calculations
- Monte Carlo Simulation - 50,000+ scenarios for dividend and investment projections
Rounding & Precision
We follow financial industry standards for rounding:
- All calculations use full precision (JavaScript's 64-bit floats)
- Intermediate values are never rounded
- Final monetary results round to nearest cent ($0.01)
- Percentages display to 2 decimal places (e.g., 6.25%)
- Large numbers use comma separators for readability
Privacy & Security
Your Data Stays Private
All thy.finance calculators run entirely in your browser:
- No server-side processing - Calculations happen on your device
- No data transmission - We never see your numbers
- No tracking - No analytics, no cookies, no surveillance
- No accounts - Use all tools without signing up
Check your browser's developer console - you'll see zero network requests during calculations.
Continuous Improvement
We regularly update our calculators to maintain accuracy:
- Annual review of tax brackets and contribution limits
- Updates to reflect current market conditions (default interest rates)
- Bug fixes based on user reports
- New features based on user needs
Found an issue? Let us know and we'll investigate.
Last updated: October 2025